Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site amdcad.UUCP
Path: utzoo!watmath!clyde!cbosgd!ihnp4!amdcad!phil
From: phil@amdcad.UUCP (Phil Ngai)
Newsgroups: net.dcom,net.periphs
Subject: Re: Definition of "null modem" cable
Message-ID: <3209@amdcad.UUCP>
Date: Fri, 30-Aug-85 02:43:29 EDT
Article-I.D.: amdcad.3209
Posted: Fri Aug 30 02:43:29 1985
Date-Received: Sat, 31-Aug-85 09:25:06 EDT
References: <195@almsa-1>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Distribution: net
Organization: AMD, Sunnyvale, California
Lines: 41
Xref: watmath net.dcom:1226 net.periphs:835

In article <195@almsa-1> control@almsa-1 (William Martin) writes:
>What is the exact definition of a "null modem" cable? Does the term
>*only* mean

1) that the connections to pins 2 and 3 on a DB25 RS232
>connector on each end are interchanged,

2) or does it also imply
>connections of other pins or the shorting or cross-connecting of
>certain pins (like 1, 7, 6, and 20)?
>
3) Or is it a general term with no exact definition, except that it implies
>that the 2 & 3 connections are crossed?

The way I look at it, RS232 is not a standard, it is just a way of saying
you are going to try to make two devices talk together. With whatever
pins work. (ever try to hook up an NEC Spinwriter to a computer?)

But in answer to your question, 3) is certainly right, 1) works for many
DEC devices and 2) is sometimes needed. If you need 2), the most general
case is as follows:

2-----3
3-----2
7-----7
4-- --4
  | |
5-- --5
6-- --6
  | |
8-- --8
  | |
20- -20
-- 
 "What really knocks me out, is a book that, when you've done reading it, you
 wish the author that wrote it was a terrific friend of yours and you could
 call him up on the phone whenever you felt like it."

 Phil Ngai (408) 749-5720
 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!phil
 ARPA: amdcad!phil@decwrl.ARPA